1. Field of the Invention
This invention relates to a distance measuring device for measuring the distance to an object and, more particularly, to improvements in the distance measuring device of a dual integration type in which a signal light is projected onto the object and the ratio of two outputs from light receiving means for receiving the reflection of that signal light (which ratio varies depending on the distance to the object) is determined by dual integration to obtain the measured value of the distance.
2. Description of the Related Art
In the so-called active type of distance measuring device which projects a signal light and receives the reflection of that signal light to obtain the measured value of the distance, there have been proposed various methods of computing the distance from the device to an object to be photographed from the ratio of two received light outputs of variable ratio depending on the object distance. For example, one method is to logarithmically compress each of the above-described received light outputs and then subtract one from the other. Another method is to A/D convert the above-described two received light outputs and then perform calculations with a microcomputer. Of these methods the ones using dual integration disclosed in U.S. Pat. No. 4,720,723 and others have the advantage that one integration circuit is sufficient, thus reducing the circuit scale, that because one circuit is time-divisionally used in common, and the problem of matching the circuit constants disappears, etc.
Meanwhile, in compact cameras employing devices of this kind, as the capability of changing the focal length of the photographic lens in discrete values or of zooming is introduced, the range of distances to be measured is widened. Hence, a distance measuring ability from a farther distance to a nearer distance is desired. This means the dynamic range of the signal processing circuit must be widened. The reason is that the signal strength varies in inverse proportion to the square of the distance to the object. Now, suppose the circuit is designed to be able to perform calculations with the signal strength at the longest distance to be measured, in other words, at the time of the weakest signal. Then the near distance object may saturate the circuit. Conversely when the circuit is so designed as not to saturate at the near distance, the signal becomes weak on long distance objects, the calculations cannot be performed accurately and reliably, causing production of errors in the distance measurement. Particularly, an error in that circuit which performs signal computation becomes a distance measurement error. Hence its influence is very great.
A so-called automatic gain control (AGC) circuit may be provided for automatically varying the mu-factor of the signal circuit depending on the signal strength. But, usually a circuit of this kind has the problems that because of the necessity of a feedback network, the circuit becomes complicated, and because the AGC operation must be carried out before the distance measuring operation, the distance measuring time is increased by that amount, etc.